1. Field of Invention
The present invention relates to a radio-controlled timepiece and its method of changing the waveform discrimination standard.
2. Description of Related Art
Germany, Britain, the United States, Japan, and other countries transmit long-wave standard time signals carrying time information on a carrier wave frequency of several 10 kHz, and radio-controlled timepieces that receive and use this standard time signal to adjust the time have become common. The long-wave standard time signal is a pulse wave that denotes a time code of pulse values such as 1, 0, and M each second, and transmits one full frame in 1 minute. Each frame contains time information including the year, hour, and minute.
The radio-controlled timepiece therefore detects the pulse waveforms denoting each code from the long-wave standard time signal, and determines the value, such as 1, 0, or M, represented by each pulse (referred to below as a waveform discrimination process).
The frequency of the carrier wave and the waveforms of the pulses denoting the code values 1, 0, or M in these long-wave standard time signals differ depending on the country.
The process whereby a radio-controlled timepiece sets the time using a long-wave standard time signal is described briefly below.
The standard time signal is received by an antenna such as a tuning bar antenna and passed to a demodulation circuit.
The demodulation circuit includes an AGC (automatic gain control) amplifier, a narrow-band bandpass filter using a crystal oscillator, a rectification circuit, and a decoder circuit.
The demodulation circuit boosts the antenna output to a required level by the AGC amplifier, extracts the required signal band using the narrow-band bandpass filter, and detects the signal by amplitude-modulated wave detection using the rectification circuit. The detector output is then compared with a reference level by a decoder, and the level is converted to output the time code signal.
The radio-controlled timepiece applies the waveform discrimination process to the time code signal and identifies the code. The time is then adjusted based on the result of the waveform discrimination process.
Each pulse in the time code of the standard time signal transmitted in Japan, for example, starts at the rising edge, that is, where the time code signal changes from LOW to HIGH. The waveform discrimination process taught in Japanese Unexamined Patent Appl. Pub. JP-A-2003-222687, for example, therefore uses a 32-Hz sampling circuit to sample the time code signal for 1 second from a detected rising edge and acquires 32 samples (samples 0 to 31).
The 32 samples are then divided into plural discrimination periods in which the HIGH and LOW signal levels differ according to the code. The Japanese JJY standard time signal, for example, is divided into period A (samples 1 to 5), period B (samples 8 to 13), period C (samples 18 to 23), and period D (samples 27 to 31).
The HIGH/LOW signal level of each code changes in the samples not belonging to any of the discrimination periods. For example, the 0 code changes from LOW to HIGH between period D and period A, and changes from HIGH to LOW between period C and period D.
Based on the number of HIGH and LOW samples in each period, the HIGH or LOW value of each period is determined.
For example, whether period A denotes a HIGH or LOW signal level is determined according to the number of HIGH and LOW signal levels in samples 1 to 5.
Each code is then identified from the HIGH or LOW signal level of each discrimination period.
If period A is HIGH, period B is HIGH, and period C is LOW, a code value of “1” is identified, for example.
Note that period D must always be LOW, and an error is therefore returned if period D is HIGH.
However, deviations can occur in the reference level of the decoder circuit and variations can occur in the capacitance connected to the AGC amplifier during the radio-controlled timepiece manufacturing process.
When such variations are introduced during radio-controlled timepiece production, the radio-controlled timepiece can output different time code signals even through the same long-wave standard time signal is received in a reception environment that is unaffected by the field strength or signal/noise ratio (S/N) of the long-wave standard time signal. More specifically, the signal width of the pulse waves in each code may vary.
When the effects of the field strength and S/N ratio of the long-wave standard time signal are considered, the signal width of the pulse waves in each code can vary even more.